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  • Here we explored the relative contribution of


    Here we explored the relative contribution of D1-like and D2-like receptor subtypes in the control of glutamate release, as well as their pharmacological properties and their interactions modulating the release. The pharmacological parameters (EC50 and Emax) of dopamine for the different types of receptors were obtained from dose–response curves. We found that dopamine stimulates D3R with greater potency (EC50) than D1-like receptors; however, D1-like receptors modulate release in a higher amount, as shown by the higher dopamine efficacy (Emax). Pharmacological antagonist effects in release from reserpinized slices treated with exogenous dopamine vs. non-reserpinized slices were similar, indicating that endogenous dopamine produces similar effects on the receptors. These findings indicate differences in the control of glutamate release by D1-like, D2R, and D3R, which could contribute to the understanding of interactions of selective agonist/antagonists controlling motor behavior.
    Experimental Procedures
    Discussion Studies of radioactive neurotransmitter release in Heparin slices have shown to be useful to estimate the effect of dopamine agonists and antagonists on the presynaptic receptors modulating neurotransmitter transmitter release, as well as their mechanism of action. In fact, the majority of these findings have been reproduced by other techniques, including electrophysiological ones. Here, to understand better the dopaminergic control of glutamate release, we utilized high K+-induced depolarization to evoke [3H]-glutamate release in rat substantia nigra, exploring the following three aspects: 1) the pharmacological profile of dopamine and dopamine-selective agonists; 2) the relative contribution of D1-like and D2-like dopamine receptors and their subtypes, and 3) the functional interaction between these. Our data indicate that, as previously described, D1-like (probably D5), D2R, and D3R subtypes modulate glutamate release from subthalamo-nigral terminals; however, their relative contribution in the control of the release is different. D1-like and D3R stimulate and inhibit the release, respectively, in a greater amount compared with the inhibition produced by D2R. D4R does not participate in modulating the release. Dopamine modulates release by D3R activation with higher potency (EC50) compared with that of D1-like and D2R. The three receptors modulate release, interacting with each other as follows: D3R and D2R antagonize D1-like receptors, while D3 and D2R exert an additive effect.
    Acknowledgments Supported by a grant (152326) from CONACYT, México to B.F.
    Introduction The classic theory of signaling transduction cascades acting via G protein coupled receptors (GPCRs) assumed that a ligand activated monomeric receptor protein causes stimulation or inhibition of the receptor actions which only depended on the kind of compound and receptor protein. Numerous studies provided evidence that signal transduction is much more complex and many proteins are able to interact with membrane receptors affecting signal transduction, and also that GPCRs can act as two-protein or higher order protein complexes [1]. Although experiments which provide evidence for GPCRs oligomerization can be easily misinterpreted and frequently require very critical analysis, at present not only the ability of GPCRs to take on oligomeric forms is being discussed, but more often, its physiological implications as well [2], [3]. Dopamine D1 and D2 receptors belong to GPCRs exerting opposite actions on the level of cyclic AMP formation, but plethora of behavioral, biochemical and electrophysiological studies provided evidence that these receptors act in concert [4], [5]. It has been shown that a part of central dopamine D1 and D2 receptors are colocalized in rat and human neostriatal neurons [6], [7], [8], [9], [10], [11], [12], [13]. This discovery was used in our studies as well as by others in studies applying heterologous expression systems in vitro. Experiments using the FRET (Förster Resonance Energy Transfer) phenomenon showed that in HEK 293 cells transiently expressing both receptors, these proteins are in close proximity (which has been interpreted as an ability to form heterodimers) that is affected by ligand co-stimulation [14], [15]. In recent years, a novel Heparin signaling pathway, activated via the dopamine D1-D2 receptor heteromer has been identified. This pathway is linked to intracellular calcium release as an effect of D1–D2 heteromer interaction with the Gq protein. As a result, phospholipase C and IP3 (inositol 1,4,5-tris-phosphate) receptors are activated [9], [10], [16]. The consequences of this signaling pathway are not known, but there are suggestions that it may be involved in the etiopathology of schizophrenia. This presumption comes from the fact that both increased dopamine transmission and abnormal calcium signaling have been linked to this neuropsychiatric disorder [17], [18]. The existence of D1–D2 heteromer in neuronal tissues and its coupling with Gq signaling started investigations indicating direct association of calcium signaling and dopamine D1–D2 heteromer with schizophrenia [9], [12], [19], [20], [21], [22], [23], [24].